texts: Deciding optical factors between a refractive and reflective space telescope optics as a function of aperture? (visible light)

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Reading Yale News' Lighting a path to Planet Nine:

To detect objects that are otherwise undetectable, Rice and Laughlin employ a method called “shifting and stacking.” They “shift” images from a space telescope — like moving a camera while snapping photos — along pre-defined sets of potential orbital paths. Then they “stack” hundreds of these images together in a way that combines their faint light.

They used Transiting Exoplanet Survey Satellite (TESS) data presumably more for it's large number of exposures of a given field than each camera's 10 cm effective aperture.
Apart from mechanical and materials aspects such as (but not limited to) mass, thermal management and radiation and meteorite damage, what optical performance considerations would to into choosing between a refracting and a reflecting space telescope for a given diameter aperture?
A camera lens like TESS' can have many more optical surfaces than a reflecting telescope. The cross-sectional image of TESS' camera shows seven elements and therefore 14 surfaces that can be optimized for near diffraction-limited resolution over a wide field of view, whereas even the Vera C. Rubin Observatory (LSST) has only three reflecting and four refracting surfaces.
Refracting glass surfaces can also be smoother than aluminum on glass surfaces at the nanoscale, reducing haze and scattered light which can impact limiting magnitude especially if there are bright objects nearby.
Question: What are the deciding optical factors between a refractive and reflective space telescope optics in visible light as a function of aperture, apart from mechanical and materials aspects such as (but not limited to) mass, thermal management and radiation and meteorite damage?
Related:

What are the benefits of a large refracting telescope?
What allows WFIRST to have similar resolution to Hubble over a 100x larger solid angle?
What are the effects of using non-spherical lenses in refracting telescopes?
See Dragonfly in What (actually) is the " deprojected half-light radius" of this almost-all-dark-matter Galaxy?
Why does LRO's laser altimeter telescopes use lenses instead of mirrors?
What's the largest optical lens put in space?

From What causes these cross-shaped artifacts in TESS' first images? (the answer is interesting if you like CCDs)


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I have no formal training in physics, just a casual interest, so if this question doesn't make sense i would love to understand why.

I know from casual physics talks etc that pairs of particles pop in and out of existence due to fluctuations in the underlying relevant field.

Does that mean that for every instant there is an extra amount of mass per unit of space equal to the massive 'fluctuation' particles that still did not pop out of existence/annihilate?

If that holds true has anyone measured it or calculated it?


texts: What is the abbreviation for solar time, hour angle of the sun?

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I understand that the hour angle and declination can be abbreviated as (HA Decl), and rectascension and declination as (RA Decl). The hour angle of a star is of course zero hours at the meridian.

The Sun however does have solar time 12 hours at the meridian, and the "solar" hour angle would better be 12 hours. Therefore I look for an abbreviation for use in computer programming for the pair (solar time, declination) or (sun hour angle, declination), and I can't make up them. I didn't anywhere find them naturally, say in Wikipedia or through Google.

I considered (SOL Decl), (SHA Decl), (ST Decl) or the german (SZ Decl). I don't want to use (HA Decl) for the sun, because HA is zero at the meridian, but as already said for the sun its hour angle should be 12 at the meridian and therefore for clarity and for programming purposes, another designation is needed.

My angles are local, there is no need for a compulsary prefix L- (local) or G- (Greenwich).

I know that in english usage is "apparent" sun hour angle, but in german it would be "true" sun hour angle, therefore the word element "apparent" should be absent.

The question is not about the formulas proper for hour angle or solar time.

Thank you.


texts: Could there be a M-type asteroid beyond Jupiter?

texts: Is there any reason to think an M-type asteroid could not have migrated beyond Jupiter and entered an orbit similar to the Centaurs?


texts: Help identifying a very slow-moving object during meteor shower photography

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I just wonder if anyone could shed any light on this puzzle.

Two nights ago whilst I was out photographing the Lyrids meteor shower from the UK something else showed up on my images when I checked them the next morning. All images were 30-second exposures at ISO 2000 shot throughout the whole night with the intervalometer. It takes roughly two minutes for a satellite to pass through an image, so that's four images which you can see when viewing the images.

This started just after 01:00 and disappeared from the frame at 01:25.

Upon going through further I have a small speck starting from the top of image which continues through the image for 41 images; doing the maths, that's 20 minutes of something passing through the sky slowly.

It never continued all the way through, just slowly got darker. I was obviously set up towards the star Vega and the Lyra constellation. Any help would be greatly appreciated.

Cropped and rotated, showing broken trail:


texts: How long will it be until the nearby groups of galaxies are receding at speeds faster than light?

texts: And subsequently how long will it be until we can never see them again?


texts:
Howdy, this might be an odd question, but I'm using whatever information I gather here to help write an alternate universe version of the 1950s through 1980s. Due to some convoluted plot contrivance involving several delays to the moon landing, the very last pass behind the moon the Apollo crew takes place during a lunar eclipse. Now, I unfortunately don't have access to any simulation software, so I'm not sure what that would look like to them! How much of the lunar surface would the crew be able to see in passes that occur beforehand, and what does the phenomenon look like from space? Do we know what it might look like?
Time is not of the essence, but I still need to know these details before writing this particular section of the story.


texts: Have we ever seen the formation of a blackhole?

texts: So I know that the formation of a supernova has been observed and that we have taken images of blackholes, however, has the formation of a blackhole ever been observed. How do we observe it?


texts: How to manually locate a certain declination in the sky for a given location? (no sextant available)

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Consider this example. I am standing in a flat surface, at latitude X, looking to the equator, in a random day of the year. (For simplicity, let's assume X > 23.5º)
I want to find the position of the sun at noon on the winter solstice (that is, the highest point in the sky of the sun, the day which this reaches the lowest altitude), location which has a declination of 23.5º.
We know the sextant does exactly that. But say I don't have a sextant. I want to find an alternative method to do the same.
If my latitude is X, then such position of the sun is X-23.5º above the horizon (looking North). I could then divide this result by 90, and get a proportion of the sky between the horizon and my zenit where that point is. But this is too rough.
Perhaps one way would be to have a long stick and a measuring tape. For a given value of X-23.5º there must be some relationship between my eye, the vertical height of the stick and my horizontal distance to the stick which allows me to find exactly the position in the sky.
Or perhaps another method?


texts: Sundial (solar clock) in a binary star system

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What would a sundial look like in a binary star system? What is the path drawn by the shadows?

It probably depends on the system, the sizes of the stars, their distance... If you know a binary star system for which a sundial would look different from what we have on Earth, we can take it as an example!

Edit: thank you to barycarter for bringing to attention that the Kepler binary stars would look like one star from the surface of their planets. However, there must be, somewhere, a binary system where both stars are visible, such that the shadows are affected.

EDIT2 : A Google search shows that this question has been answered before, but all links to the answer are dead. - I searched "binary solar system clock", found a link by a "Charmaine Pauls" -
But after using "sundial" instead, I found this pintrest artwork, but it is not much help. It doesn't seem to be scientifically motivated.


texts: Relationship between photometric colour and redshift

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In this paper, the authors describe the theoretical relationship between photometric colours, particularly in Figures 1 and 6:

Since the colour is the difference between two magnitudes, and the redshift is defined as
$z=\frac{\lambda'-\lambda_0}{\lambda_0}$

I would have thought that the colour-z relationships would simply be a downward curve, since the light gets redshifted (goes down in frequency), and the ratio of two magnitudes is constant with redshift.

Why do the colour-z relationships in this paper (and in others I've seen) show such a complex relationship?


texts: Why smaller black holes are considered to be more dangerous than bigger black holes?

texts: More massive the body is more is the gravity.Still often smaller black holes are considered to be very dangerous than the bigger ones,why?


texts: What is needed to break through the surface of Europa?

texts: How big does a satelite we send have to be to be able to chrush through the ice layer and dive into the ocean on Europa? If a mission should head there, is this a better way of examining the ocean than landing on the surface and drilling through?


texts: If a black hole pulls every object in the universe, why there is a light around it?

texts: I've seen some black hole images and I've seen some movies about it e.g(interstellar). I wonder why it has a light around it. Even if I know that light can't escape through it. Can someone enlighten me about this? Thanks.


texts: Examples of Equal Mass, Unequal Mass and Double Binary Star Systems

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Can anyone give me an example of a star (ideally those with a common name e.g. Rigel, Sirius, etc) of the following multiple star systems.

Equal Mass Star System, where two binary stars orbit round a central point of space.

Unequal Mass Star System, where a smaller star(s) orbits round a larger binary star.

Double Binary where two sets of binary stars orbit round a spot. (Regulus?)


texts: How many moons do we know are shepherds?

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Shepherd moons are natural satellites near the edge of a planetary ring that have an influence on the shape and size of the ring due to their gravity. So it is possible for a satellite to be near the rings without being actually a shepherd. For instance, Pandora is on the outer side of Saturn's F ring, but isn't involved in shaping the ring, so it isn't a shepherd. On wikipedia, it says that we can't know if Cordelia and Ophelia are shepherds or not (I haven't found the reference for that yet, though) because we need better observations.
How many satellites in the Solar System do we know are shepherds?


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Hello fellow amateur and professional astronomers,

On July July 17, 2017, I was at an observing session in Avrig, Sibiu county, Romania (45.7288° N, 24.3784° E). At approx 11:30 pm I pointed my green laser pointer (class III, wavelength 532 nm +/-10, YL-Laser 303) towards Arcturus in order to help my colleague align the SW AZEQ 5 mount within the 3-star procedure.

While the laser beam was on, at approx 1 arcminute beneath and to the left from Arcturus, with my naked eye, I observed a point of light (white/blueish) lighting up to the point which I estimate to be close to the visual magnitude of Jupiter or slightly higher for appprox 5 seconds, and then the "flare" faded out to black. As there was no "regular" star-like remanant, I was left to conclude that the initial flare did not originate from a star.

I ruled out a reflection of the laser beam from a satelite (as too far for my laser's power), a plane or a bird (due to the white/blueish light I saw, its fixed position and apparent distance to my eye), as well the possibility of a meteorite burst (as it had no apparent trajectory, but it appeared as a fixed point).

I am so intrigued and curious as to what possible optics or physics could have been involved and explanations or ideas you may have to explain the phenomenon?

I searched online for others' possible similar observations without success.


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So I had an argument with a friend, which was about whether the Moon is a planet or a satellite.

IAU 2006 Resolution B5 gives definitions on what it is to be a planet, but there is a vagueness, as I perceived, is that it does not define satellites, while using it to define dwarf planets.

There is a supplementary Q&A sheet, and it uses the barycenter to define a satellite, but it is implied, not explicitly defined.

My questions follow:

Should the supplementary Q&A sheet considered as legitimate official definitions, or should it be considered as a one way of interpreting the Resolution B5?
If Q&A is legitimate, then why is the definition of the satellite is not included in the Resolution B5? If not, is the official definition of the satellite nonexistent currently?
Are there any source or archive of the official or widely accepted definitions of glossaries in the fields of astronomy?


texts: A greenish blue patch in the sky during sunset

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I observed a greenish blue patch in the red sky after sunset, and it stayed for not more than 5 minutes. It didn't curve or appear distorted; it was just like a carpet. Why is it so? Could it have been an aurora?

Thank you

location: 9° N, 78° E; Temperature 35 deg C.


texts: How long does an over contact binary star system last?

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I read recently about VFTS 352, an overcontact binary star system where both stars have roughly equal mass. All of the reports I've read (in mass-media type publications) have said that the system has one of two fates: either the two stars will merge, or they'll supernova. But when will this happen?

The wikipedia page for contact binaries says that they have a lifespan of millions to billions of years, but doesn't say if that's different for overcontact binaries. It also says that they're often confused with common envelopes, which have a lifespan of months to years, and I'm not sure where in that spectrum an overcontact lies (or really what the distinction is, since the page for contact binaries says they share an envelope, which sounds the definition of a common envelope). I'm also not sure whether the fact that both stars have roughly equal mass affects the lifespan.

The mass-media articles I've read have implied that the merger-or-supernova is happening soon, but I don't know if this is on a human scale (months) or galactic scale (millions of years).


texts: (Thought Question) What would a giant far away mirror look like to a telescope?

texts: If there was a theoretically perfect mirror the size of say, our solar system, somewhere out in space that had a focal point of literally earth. What would that look like to a space or earth telescope? Could it be distinguished from other parts of space? Could you identify earth in it? (A la By putting a mirror in space, would we be able to see into the past?)


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I've read that a gas giant, like Jupiter, doesn't have a rocky surface. But planets must start as a rocky conglomerate of flotsam.

Is the interior so hot that all the material is molten and there is not a crust?
Does the atmosphere creates enough pressure to create a supercritical fluid?

What is the situation on Jupiter?


texts: North node, South node correct understanding

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What is the correct concept of the North node and South node? There is some conflicting information on the internet.
Which one of these two is correct?
1. Sourced from some Astrology material - How can North and South be 180-degree opposites in this since these are on one side of the orbit curve?
https://astrologyquestionsandanswers.com/2016/10/12/major-and-minor-chords-the-moons-nodes-part-two/

https://www.spiritdaughter.com/blogs/align-with-magic/a-shift-in-perspective-the-north-node-moves-to-gemini

http://astrologyclub.org/eclipses-and-the-moons-nodes/

2.From Wikipedia source - This seems incorrect too since there is no "separate" Moon orbit as per around the Sun
https://en.wikipedia.org/wiki/Lunar_node

http://www.antidotebrooklyn.com/celeste-blog/2019/4/21/riding-the-tail-of-the-celestial-dragon


texts: Can lunar occultation of Venus occur during solar eclipse?

texts: Moon can occult Venus. Venus can transit across the solar disc. Moon can elipse Sun. Can all three celestial events occur at the same time - Moon is at front of Sun (partial eclipse is fine) and Venus is behind Moon (ideally part of the planets is seen at lunar limb)? Observer is at Earth.


texts: Why are only some of the geologic features in Mars' Noctis Labyrinthus region named?

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I found this page Names Approved for Six Cavi and a Tholus on Mars which lists seven features on Mars. I'm curious about the decisions to name only these features: Dalu Cavus, Layl Cavus, Malam Cavus, Nat Cavus, Noc Cavus, Usiku Cavus, and Noctis Tholus in Noctis Labyrinthus. Why only some of the canyon, graben and/or valley locations are named?  Does it have to do with the depth of the features?

Doing a search for the area of Noctis Labyrinthus, Northernmost Latitude: -3.0, Southernmost Latitude: -13.0, Westernmost Longitude: 259.0, Easternmost Longitude: 268.0, I found only this,


texts: How to calculate center (not focus) of planetary orbit?

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I'm developing a website showing the movement of Solar System planets according to the passage of time. I'm using this reference to calculate all the necessary orbital parameters.
These are two images some days apart.

(Only planets up to Mars are shown here. The moon orbit has been highly exaggerated to become visible. Celestial bodies radii are not to scale either, except among themselves—except the Sun.)
The images show the high eccentricity of Mercury's orbit, causing it to be very far from the Sun in the first image, and very close to it in the second.
I want to draw the correct ellipse, instead of circles that constantly change radius. I have all the “orbital elements” given in the link above (N, i, w, a, e, M, r, v, x, y, z, lon, lat), but don't know how to extract an ellipse from them. I'm drawing the planets using x, y heliocentric coordinates. Even if I had the ellipse foci, JavaScript needs the geometric center of the ellipse to draw it, and I don't know either how to convert the former into the latter.
How to calculate the ellipse center from the parameters given above?


texts: Is there a reason two interstellar objects were found in the solar system in such a short period of time?

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ʻOumuamua was discovered on October 19, 2017, and 2I/Borisov was discovered on August 30, 2019.  Has something changed about observation techniques or equipment that made it possible to find these objects in less than two years?

I find it hard to believe that there has been no interstellar objects passing through the inner solar system in the past ten or twenty years, then suddently these two did, quite close in time.

I recognize it is incredibly hard to find such small, dark objects moving so fast, especially if they are outside of the ecliptic, and there be many more passing through that just haven't been discovered. Should this be chalked up to pure luck?


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Some X-ray sources in nebulae are caused by hot materials in the millions of degrees that formed in supernovae. What's the cooling rate of this stuff? How long does it stay this hot?

If we would take our solar system for example, the time from the formation of the nebula (by a supernova?) until it was cool enough to form solid planet was only a few million years. That's very quick, for instance relative to the age of the Earth. How ephemeral are these X-ray sources? I would expect them to not last as long as IR sources.

Is there an order of magnitude for the lifetime of the emission? Does it take millions of years? Tens of millions? Tens of thousands of years?

A comment suggests it doesn't really cool down, but diffuse to the interstellar medium. How long does it take for it to diffuse until there isn't a clear X-ray source?


texts: How can we tell that the sun is moving with theories such as the theory of relativity?

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In a nutshell, the theory of relativity says that we cannot know that the ether is moving because if we were on the sun, the earth would seem to be moving and visa-versa. Apparently, we can't find absolute motion; we can only find relative motion.

How do we know that indeed the earth moves?
What is relative motion?
What is absolute motion?


texts: How significant are the recent results that Scholz’s Star has perturbed several observed hyperbolic objects?

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The Gizmodo article A Visiting Star Jostled Our Solar System 70,000 Years Ago refers to the the three year old ApJ. paper The Closest Known Flyby of a Star to the Solar System (also arXiv) which describes Scholz’s Star and it's pass close to our solar system about 70,000 years ago.
The Gizmodo article then discusses the newly published paper in MNRAS Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies. However, while I can access the earlier paper, the new result is paywalled and I'm not able to access it (at least not soon).
The second half of the abstract in particular is confusing me.

Here, we perform a systematic numerical exploration of the past orbital evolution of known hyperbolic minor bodies using a full N-body approach and statistical analyses to study their radiants. Our results confirm the theoretical expectations that strong anisotropies are present in the data. We also identify a statistically significant overdensity of high-speed radiants towards the constellation of Gemini that could be due to the closest and most recent known fly-by of a star to the Solar system, that of the so-called Scholz's star. In addition to and besides 1I/2017 U1 (‘Oumuamua), we single out eight candidate interstellar comets based on their radiants’ velocities.

I've heard of a radiant of a meteor shower - it's an apparent position of the source of meteors referenced to the position of the Earth at the time of the shower. But I've= never heard the term applied to the orbit of a minor body unrelated to the Earth's position. What is the "radiant" of a minor body with a hyperbolic orbit in a simulation? Could it be the asymptotic incoming direction of the hyperbola?
But my main question is simply how "statistically significant" is this result in reference to the enhancement in the direction of Gemini? I can't see the data. Is it just a tiny fluctuation, or a fairly obvious and robust feature?
A screen shot of the data would be most welcomed!
Possibly helpful: List of hyperbolic comets.


texts: Model/Infographic of plane of orbits and rotations planes of solar bodies (including their moons)

texts: Is there a model/infographic with emphasis on the orbit and rotation plane of solar bodies? Of course, no scaling is needed for the size or distance. I remember reading there is one planet where the axis of its rotation is almost parallel to plane of its orbit.


texts: Is it a coincidence that both the Sun and the Moon look of same size from the Earth?

texts: The Sun is huge, when compared to the Moon. Despite the huge difference in their size and distance from Earth, is it purely coincidental that they both look almost the same from Earth?


texts: If the moon wasn't receding from Earth, what would be the impact on the weather and tides?

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Moon is slowly receding from Earth, which means that after its formation it was much closer to the Earth than now. How would weather look like if it wasn't receding at it would be now as close as at the beginning?

Surely, the tides would be much higher, but how big would be the difference? I've also watched document stating that the hurricanes would be much stronger. Was there any simulations, how much stronger they would be?


texts:
So I recently got into astronomy and I am using my fathers old Meade EQ1-B reflector telescope. This is my first night using it and I have come across a few questions. To begin, the telescope was left alone for several years (the scope is a fairly old model) and may have become damaged over time, yet nothing too severe. We were cleaning it and got to the mirror to find that over the years the "mirror" cover on top of the glass had chipped away all over, giving the result of a patchy look.

We were testing the scope out during the day and the image was fine - the mirror didn't affect it. Now, I am looking upon stars and have noticed a couple of things. When viewing these stars the image sort of moves, not the scope, the image. As if it were an animation, the star isn't just a white blob, it looks like the sun, where the blob is moving - as if it were a liquid. No I am not stupid enough to point the scope at the sun... during night time. The stars just move like a liquid in the center, is this an effect of the mirror? I can't see why it would be as the image was fine during daylight. Is this how all stars work? I haven't had a chance to point it at the moon or any other planets so I can't test it on an object that isn't a star.

Also, as the scope is a reflector it has the "three lines" on the front, however this interferes with the object I am looking at as i can see the lines on the star. Is there any way to stop this, or is it just the result of using a reflector?

Thanks!

Image below:


texts: How fast are we moving relative to the CMB?

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The cosmic microwave background radiation should provide kind of a global reference frame, because you can determine your speed relative to it using the redshift.

Is it known how fast we are moving in relation to the CMB? If you subtract the various orbital motions (Earth around the Sun, Sun around the Galaxy), are we standing still in the expanding universe, or traveling in a certain direction?


texts: Light Pollution: what is the difference between artificial brightness and brightness and SQM

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At this light pollution map they show degrees of light pollution but they have 3 numbers to quantify light pollution and they do not explain in the help section what the numbers mean:
The numbers are SQM, brightness and artificial brightness.  I get that for the last two the lower the better for viewing stars, but I can't figure out if brightness perhaps includes artificial brightness.


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If space is a vacuum, then how can it contain an odor? I read that after spacewalks, astronauts detected an unusual odor after removing their spacesuits.  It was compared to the smell of a candle burning. Perhaps this is the smell from the "Big Bang"?

Here's a former astronaut describing the smell: Astronaut Chris Hadfield Debunks Space Myths | WIRED "This one is true! (gunpowder, burnt steak, brimstone)" and What does space smell like? Astronaut Chris Hadfield answers your questions "a little bit like a shooting range, gunpower/cordite, brimstone"


texts: Have any planetary systems been found around Population II stars?

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I always understood that Population II stars were unlikely to have planets as they were virtually all H and He with very little else.

Have recent observations confirmed this, or have planets been discovered around Population II stars?


texts: What are the 4 candidates for Planet Nine reported in 2017?

texts:
From Wikipedia Planet Nine#Citizen science:

  In April 2017, using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.

But it has not been updated for nearly 3 years. Is there any follow up study that confirm what these 4 candidates actually are?


texts: How did astronomers first come to understand that the Sun's core was a gas?

texts: I was reading a very old book (around 1850) on astronomy and the author explains Sun spots as a phenomena where you can see the black planetary like hard core of the Sun through two different atmospheres.  It got me interested that somewhere between 1850 and now it became understood that the Sun's core is a gas.  How was this determined conclusively and approximately when in history?


texts: Did we ever actually see the earth revolving around the sun? Is the geocentric model completely disproved?

texts:
Did we ever actually see the Earth revolving around the Sun?

Also, is the geocentric model completely disproved or was it sidelined because the heliocentric model made things easier to understand?

(BTW I know Earth revolves around the Sun and am just asking out of curiosity.)

Thanks!


texts: Relation between intensity of Sun and the light that is emitted from surface of sun

texts: please correct me if I am mistaken, I was in the middle of a thought process. We take a unit area dA of sun,, which is emitting light directly to our line of sight. But there are also numerous other unit areas throughout sun's surface whose light does not reach our eyes directly. Then why do we still see the sun with same intensity?


texts: How to find if a set of objects lies within a given resolution (5 arcsec) on a telescope?

texts:
I am new to astronomy and I was given the task of finding out if there are objects which can't be resolved by a telescope of resolution 5 arcseconds.

I was given the values of RA and Dec of these objects in degrees.

   RA              DEC
 201.1999388    27.49294258
 201.2048319    27.49212959
 201.1984702    27.4978016
 201.2025157    27.51610266
 201.1997827    27.51743447 ...etc

How can I find these in the simplest way? Is there any simple formula which can be used to achieve this which is computationally efficient as well?


texts: What would Earth be like without the Moon?

texts:
If somehow our moon was destroyed (for example by an asteroid) what would happen to Earth?

Would the Earth be affected by the absence of the moon? Would humans be able to live normal lives without the moon?


texts: What type of star does theory predict should be the most massive?

texts: The most massive known stars are Wolf-Rayet stars. However, as Wolf-Rayet stars do not appear to be the first stage of any star's lifecycle, I infer that whatever these Wolf-Rayet stars used to be must have been even more massive. My guess is that the most massive stars are Wolf-Rayet because of some combination of 1) their luminosity and/or 2) the potential short life of prior stages of the star's evolution. Assuming this is correct, to what group would we expect the most massive stars to belong (even if we don't know of specific examples)?


texts: How much would you weigh if Earth stopped orbiting the Sun?

texts:
I have a curious question.

The reason why astronauts are weightless in space is because they are orbiting Earth. If they suddenly stopped orbiting they would fall directly into Earth. Earth is doing the same around the Sun, so it would fall directly into the Sun if the Earth stopped orbiting the Sun.

If the Earth stopped rotating and orbiting, obviously ignoring the adverse effects of doing this and having Earth hold it's current position in it's orbit, how much would a 150 pound person weigh if they stood on suitable position on Earth.

This question is kinda out there, but it's a thought-provoking question for me and hopefully others.


texts: How to calculate sunrise and sunset on on a given date and location?

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I have a given date at any time in future or past eg: 2016-04-23 or 2022-05-12
Now given at any location(lat,lng). How do i find the time of sunrise or sunset.


texts: What is the difference between spicules and prominences/filaments. I know that they are both in the chromosphere, and I know the difference between provinces and filaments (depends on the view), but how are spicules different?


texts: Solar Telescope Projector Screen? [& intro telescope for solar eclipse]

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I don't know very much about telescopes; I used one a few times as a kid, but that's about it. I understand the general design & use of reflectors & refractors, but I don't know enough to decipher focal length, etc. So, I don't feel comfortable trying to DIY my own contraption, that won't be nearly as good, assuming it works at all.

I'm looking to buy a minor telescope (a few hundred dollars [US] or less) to view the American Total Solar eclipse on Aug. 21, 2017; and possibly reuse for the Mercury transit in 2019. I'm a decently well off computer programmer, so blowing $200 (& some gas to drive up-state) for a one off just to experience the Solar eclipse is worth it to me, even if I don't use it again.

I spent a few hours wandering around the internet for something, and I found a few styles of "Projection Screens/Scopes" that are really cool, but I can't find anywhere to buy them. The first is an image that several websites used as their article's banner image, but none say what it is or where to get it.

I found another one, where a guy hacked together a projector scope from an old 8mm film projector.

I've found many DIY tutorials to roll-your-own out of dixie cups & Pringles cans, but as I said before: I'd rather not waste my time fumbling with that, since I'd probably screw it up; I'd rather pay money to get one that's good quality & done right.

I've wanted to buy a big light bucket for years now, so I can start looking at nebulae & planets, but never got around to it; mostly because I'm not sure what to buy. I've been looking at the "Orion SkyQuest XX14i Dobsonian" for a while, but never pulled the trigger. Given that the eclipse is only 3 weeks away, I figured something smaller would be a faster choice in the short term.

I found the "Celestron EclipSmart 50 50mm f/7.2 Alt-Az Solar Telescope" that comes with a backpack storage bag, for \$100. It has a built-in (i.e. non-removable) solar filter, & a collapsible tripod. But that seems pretty cheap; is \$100 for a telescope like that normal, or is that a poor quality trap for laymen like me?

Do yall have any advice on something to get for the solar eclipse? And is there anywhere to get a professionally made projector screen like that, so I can watch the multi-hour process from a chair rather than bending over for 2 hours? (p.s. I'm 6 foot 3 inches tall) Also, if possible, being able to record video of it on my Samsung Galaxy S6 would be cool, but not required if that's a big deal.

Thanks for your help,

-Yurelle


texts: What is the LSST's plan to address frequent satellite trails in data?

texts:
update: From Nature.com News article: SpaceX launch highlights threat to astronomy from ‘megaconstellations’:

But an upcoming, cutting-edge telescope could be in bigger trouble. The US Large Synoptic Survey Telescope (LSST) will use an enormous camera to study dark matter and dark energy, asteroids and other astronomical phenomena. It will survey the entire visible sky at least once every three nights, starting in 2022. Because the telescope has such a wide field of view, satellites trailing across the sky could affect it substantially, says Tony Tyson, an astronomer at the University of California, Davis, and the LSST’s chief scientist.
He and his colleagues have been studying how up to 50,000 new satellites — an estimate from companies’ filings with the US government — could affect LSST observations. Full results are expected in a few weeks, but early findings suggest that the telescope could lose significant amounts of observing time to satellite trails near dusk and dawn.

In the 2011 Sixty Symbols video Spy Satellites (from Deep Sky Videos) after about 01:40 amateur astrophotographer Nik Szymanek says:

One thing I find when I’m out here imaging, by pointing the telescope at that part of the sky sort of south east at this time of night (early evening) there does seem to be a lot of satellites picked up by these images, and even on this 60 second exposure you can see we’ve got another satellite image there.
Well I mean satellites can be a problem for a deep sky imager in that if you’re using a telescope that has quite a wide field of view and you’re taking fairly long exposures, like five minutes or ten minutes, then at some point during that exposure it’s quite likely a satellite will be recorded passing in front of the image.
In fact a recent image that I’ve taken of the Andromeda galaxy with a refracting telescope actually recorded five satellite trails in a five minute exposure, so that gives you a good idea just how crowded the skies are with all these satellites are in orbit.

To me "using a telescope that has quite a wide field of view... taking fairly long exposures like five or ten minutes..." is a good description of how the Large Synoptic Survey Telescope (main site) will operate. Granted the exposures may be closer to 15 seconds than 5-10 minutes, but on clear nights it will be imaging almost continuously for high throughput. The FOV is about 3.2 degrees, so "wide field" applies.
Question: Has a plan or strategy been outlined or established to reduce the number of bright satellite trails that appear in LSST perpetual sky surveys, and/or to process images to remove the trails or at least flag pixel regions that contain likely trails?
Nearly every satellite and other large object in low Earth orbit is catalogued and it's position versus time can be predicted to a few kilometer accuracy (7km/400km is about 1 degree for example) and the object being in sunlight or dark can be likewise calculated, so one could conceivably adjust the exposure sequence to avoid some of the brighter ones and predict where they are likely to appear in data. So I'd be surprised if nobody has addressed this in a formal way yet.

  LSST from here


texts:
I have SED data spanning over a couple of days and I'm able to find the parameters of the SED (R, B, break energy,...) using least squares method. My question is how to model light curves (with t_acc, t_esc, ...) from the data that I have.

Example of what I want to do is in the image below (the black dots are actual observed data and the red line is the model that I want to create):


texts: From where does the energy for gravitational waves come from?

texts:
As far as I understand, in the events detected by LIGO, about 4% of the total mass of merging binary black holes was converted to gravitational waves.

Where does this energy come from, i.e. what exactly gets converted into gravitational waves?

Is it simply the kinetic energy of the merging objects (velocities of these objects before merger are huge, up to 60% of c if I recall correctly), so does it mean emitting gravitational waves makes them orbit slower, but retain their original masses? Or do the compact objects really lose "real" mass, meaning they become lighter and in case of BHs their radius changes accordingly?

As an example, let's assume two BHs, both with 50 solar masses, orbiting each other far enough (say 1 light year) so that GWs nor kinetic energy has no significance to these initial mass measurements. During the merge, they should radiate about 5 solar masses in GWs. Would the resulting black hole have mass of 95 or 100 solar masses?


texts: I've been doing some research on white holes, and there has not been enough information to provide the answer. Some websites are saying that they might exist, but it is not yet proven. So, is it true that they exist or not.


texts: Are there any known neutron star - supergiant binaries?

texts: After looking for more information about TZOs (Thorne-Zytkow Object), I came up with a question that I had trouble finding answers on the Internet. Do we know of any future TZOs, supergiants being orbited by/orbiting a neutron star whose orbit is decaying at a rate that will cause the supergiant to merge and form a TZO before it dies in a supernova? If so, what are they? If not, why can't we find them?


texts: Origin of terms "white dwarfs" and "red giants"?

texts:
Can someone explain my why "white dwarfs" and "red giants" were so named? And maybe that's a silly question but why not in the opposite way?

@EDIT
Is this somehow co-related with Stefan–Boltzmann law?


texts: What was the length of year 1 million years back?

texts:
We know that the universe is gradually expanding and this indirectly means that the gravitational force between sun, earth, planets and other stars (roughly anything in the universe) is gradually decreasing as gravitational force is indirectly proportional to square of distance between the objects.

So I think this also effects to length of the year. If yes then is it possible to know how days does 1 year had 1 million year back?


texts: How can a naked singularity be possible?

texts: How can we have a singularity, without a black hole around it? How would one form? What does it do?


texts: Is there a general term for epicycles, deferents, and eccentrics in Ptolemaic astronomy?

texts:
According to Ptolemy's (c. 150 CE) account of the motions of planets, planets moved in circular paths ("epicycles") around center points that in turn moved around the center of the earth along a path called a "deferent", or around point near the center of the earth, in a path called an "eccentric".  (I think I have this right.)

Is there a general term for these three kinds of paths?  "Orbit" doesn't seem correct, since none of the three are orbits in the modern sense.

I'd also be interested in knowing of a general term of the centers of these circles.

EDIT: I originally posted this on History.SE, because I think of it as a question for historians of science, and the History of Science SE site is in Area 51.  Someone at History.SE suggested it would be better to ask this question on English.SE.  I did, but I left the original question up at History.SE.  The question got downvoted at English.SE, and was migrated here.  There is now an answer at History.SE that I've accepted. Perhaps this question should be deleted from Astronomy.SE?  Or I should not have posted it in two places?  I'm reluctant to delete a question for which someone has posted an answer.


texts: Is Ceres a terrestrial-type (dwarf) planet?

texts: Does Ceres belong to the terrestrial-type celestial bodies?


texts: How do we know the big bang didn't happen in an existing universe?

texts: I understand the evidence for the big bang (expansion, background radiation, etc), but how do we know that it was the start of the universe? Why couldn't it have occurred in an existing, but very sparsely populated universe?


texts: The vastness of space brings me a sense of chilliness even though I have never experienced it, although I wish to. Just how cold is interstellar space (on average)? How is this even measured? I mean you can't just stick a thermometer in space, right?


texts:
So, recently I came to know that this asteroid is orbiting Earth from past 3 years and still it is orbiting. It is kind of a mini Moon to the Earth know to have come from asteroid-belt.

  Is there any threat of collision by 2020-CD3 on Earth and how long it will take to escape the Earth's gravity?


texts: What does the velocity dispersion of a galaxy mean?

texts: Also, what is the advantage of getting information about the velocity dispersion of a galaxy?


texts: Mountains are higher than the atmosphere?

texts:
Is it theoretically possible that planets exist with mountains so high that their peaks overtop the planets atmosphere? And which physical laws are relevant for this question? I'm just curious.
The question came while I imagined the atmosphere like a second ocean above our ocean of water. And I thought it would be nice if such a gaseous ocean could have islands as well.


texts: How was Henrietta Leavitt's discovery important for star measurement?

texts: Henrietta Leavitt discovered a relation between luminosity and period of variable stars, so it was possible to measure distance between two variable stars. I am curious to know how that was important back in time. How is the luminosity-period relation applied to measure distance? Was this method important because they finally had trusted apparent/absolute magnitude values to apply to the distance modulus equation?


texts: Disk-integrated vs disk-resolved, what is the difference?

texts:
I'm trying to find online what is the difference between disk-integrated vs. disk-resolved, but I can't find a website which does explain them, especially in the context of modelling spectral flux density. Also, since the word disk is being used, it is only applicable to spherical planetary bodies? Is spatially-resolved and disk-resolved the same thing?
Source


texts:
This may have a simple answer but I can't find it.

I've come across this term in a paper I was reading,  (Walmswell and Elridge, 2011) which mentions 'non-detection' of Supernovae, and while I can wrap my head around detection, I was lost at 'non'.

I tried finding a definition but to no avail. Would anyone be able to give one?


texts: Is there evidence of super-heavy elements in the x-ray and gamma-ray spectrums of neutron stars?

texts: It occurs to me, that between the surface and interior of neutron stars, gravitational pressure might produce super-heavy elements and that signatures of such elements might be detectable in the star's spectrum.


texts: What datasets containing KIC 8462852 are available online ? I've found only light curve from Kepler survey.


texts: If we consider a point on the central part of a contracting object as observed from earth, the point is continuously moving away from us as the object contracts. Moreover, light emerges from a deeper gravitational well as the object contracts. So, shouldn't contracting objects show a redshift? And if so it should be less towards the edges than towards the centre, right?


texts: Gravity of a gaseous planet without a core

texts: Both Jupiter and Saturn have rocky cores.   Is there such of a thing as a gaseous planet without a core?   And would a planet without a core have gravity?


texts: What is the relative time difference between us and a star system in outer layer of our galaxy?

texts:
Just curious to know what will be the time difference between someone living on the outer layer of our galaxy and us, considering the known facts that

Time is relative to gravity
Our sun goes around at 483,000 miles/hour(792,000 km/hr) to complete its galactic year
Star systems at outer layer travel faster than ours due to Dark Matter and Dark Energy - couldn't find speed of those star systems though!

We can assume that both (Solar system and outer layer of star system) are on the same side of the galaxy.

EDIT:
In order to re-phrase this question more clearly, as per Joan.bdm, it is related to time dilation - what is the aging rate difference if we left one of two twins on the Earth and the other one on a planet at outskirts of the Milky Way.
Conversely, what is aging rate difference between closer to center of our galaxy and us?


texts: How do we calculate heat flow or cooling rate for no fusion brown dwarf star?

texts:
I was reading this article and it said something that didn't sit right with me.

"SIMP0136 still gives off light mostly in the infrared wavelengths, as its temperature is now approximately 830° C (1,526° F)," Gagné told us. "It will take it approximately 10 billion years more before it reaches 0° C"

SIMP0136 was classified as a brown dwarf star but when they just recently figured out its mass to be only 12.7 times that of Jupiter they downgraded it to a gas giant planet.  The threshold to qualify as a brown dwarf is 13 Jupiter masses.

Like gas giant planets, brown dwarf stars do not gave enough mass for fusion to occur so the term star is a bit misleading.  Since there is no fusion, then it should eventually cool down.  The 10 billion year cool down time to go from its present 830 deg C to 0 deg C seems way too long.  How do they calculate this?


texts: How massive does a moon have to be to have a stable magnetic field?

texts:
Is there a minimum mass or other minimum properties necessary for a body to have a strong, stable dynamo to create a magnetic field conducive for life?
For example, would it be possible for Titan to have a magnetic field if it had different core characteristics?


texts: Could Venus or Mercury have a moon that we haven't detected?

texts:
Let's consider Mars' two moons for a moment. They're small, not even spherical, and most likely are captured asteroids. They weren't discovered until the late 1800's. Telescopes are much more powerful today, so you'd think we'd have found any similar moons at Venus or Mercury by now, if they exist.

The trouble is, there are some very dark asteroids out there. And of course, there are some much smaller ones too.

How do we know Venus or Mercury doesn't have a small dark asteroid moon? Did any probes actually try radar to find them?


texts: Determination of orbital elements for Trans-Neptunian Objects, how?

texts:
How are orbits calculated for Trans-Neptunian Objects?  I understand there are six standard parameters for defining an orbit (link) but I am not sure how telescope observations translate into these parameters.  Telescope observations tell us where the object is on the Celestial Sphere but can't even tell us (without a good spectroscopic line to do a redshift analysis with) how fast the object is moving towards or away from us.

Additionally, the motion of these objects on the sky is dominated by Earth's motion relative to the objects rather than the motion of the objects themselves.  This seems to me to offer additional complications.

My question is: how do 2-D observations get fit with a reasonable degree of accuracy to a 6-D parameter space with the additional complication of Earth's motion relative to the object?

Don't be afraid to be mathematical in your response.


texts: How to determine the mass of a runaway star?

texts: I would like to know how do astronomers estimate the mass of a star that has been ejected from its galaxy.  What kind of accuracy can be achieved?


texts: What does "GPU-accelerated butterfly matched filtering over dense bank of time-symmetric chirp-like templates" mean? (GW170817)

texts:
A new analysis of gravitational wave (and other data) from GW170817 on 2017-Aug-17 has been published, strongly suggesting that the merger of two neutron stars resulted in a large, rapidly rotating neutron star, rather than a black hole. From the abstract of the recent open access Letter in MNRAS Observational evidence for extended emission to GW170817:

[...]Here, we report on a possible detection of extended emission (EE) in gravitational radiation during GRB170817A: a descending chirp with characteristic time-scale τs = 3.01 ± 0.2 s in a (H1,L1)-spectrogram up to 700 Hz with Gaussian equivalent level of confidence greater than 3.3σ based on causality alone following edge detection applied to (H1,L1)-spectrograms merged by frequency coincidences. Additional confidence derives from the strength of this EE. The observed frequencies below 1 kHz indicate a hypermassive magnetar rather than a black hole, spinning down by magnetic winds and interactions with dynamical mass ejecta.

Maurice H P M van Putten and Massimo Della Valle,
Monthly Notices of the Royal Astronomical Society: Letters, Volume 482, Issue 1, 1 January 2019, Pages L46–L49, https://doi.org/10.1093/mnrasl/sly166
Discussion in paper points to supplementary data and in the introduction to that document, it says:

For GW170817A/GRB170817A, we perform a model-independent deep search for broadband extended gravitational-wave emission in 2048 s (LIGO 2017) of data at 4096 Hz according to Fig. A1 comprising

Pre-processing: cleaning and glitch removal (Abbott et al. 2017a) followed by whitening of H1, L1 and V1 data;
Singe detector spectrograms by GPU-accelerated butterfly filtering of H1, L1 and V1 by matched filtering over a dense bank of time-symmetric chirp-like templates (van Putten et al. 2014; van Putten 2017);
Merging spectrograms by coincidences in frequency or amplitude, producing merged spectrograms as input to image analysis (van Putten 2018).

I am having trouble understanding what GPU-accelerated butterfly filtering of H1, L1 and V1 by matched filtering over a dense bank of time-symmetric chirp-like templates means. van Putten et al. 2014 in the supplementary data document refers to both BROADBAND TURBULENT SPECTRA IN GAMMA-RAY BURST LIGHT CURVES and to  as well. These appear to be thorough explanations, but pretty in-depth.
Question: Is it possible to explain the basics of what "GPU-accelerated butterfly filtering of H1, L1 and V1 by matched filtering over a dense bank of time-symmetric chirp-like templates" means? A dense bank of chirp-like templates sounds like it could be analogous to a wavelet-type analysis, but with basis waveforms tailored to this specific problem.
First image is a cropped and annotated version of the second which ia from here.

Figure2. Ascending–descending chirp in the (H1,L1)-spectrogram produced by the double neutron star merger GW170817 concurrent with GRB170817A (Goldstein et al. 2017) past coalescence (tc = 1842.43 s). Minor accompanying features around 100 Hz ( 1840-1852 s) are conceivably due to dynamical mass ejecta. Colour coding (blue-to-yellow) is proportional to amplitude defined by butterfly output ρ of time-symmetric chirp-like template correlations to data.


texts: Why is the detection of gravitational waves such a "Big" deal?

texts:
I'm a non-scientist. Today's announcement of LIGO's detection of GW waves has been heralded as "huge" –on the same level as the Higgs Boson detection. I realize this is the last remaining part of Einstein's theory of general relativity to be directly observed. Is that why it's such a big deal? Does this mean the theory is now on pace to become the  law of general relativity?

Side note: I had the privilege to visit and take a general tour LIGO in WA in 2000 or so and found it quite fascinating from a layman's pov. I've been waiting for this day ever since.


texts:
I thought dark and anti matter were kinda the same, but after saw a video, they mention that dark matter is not antimatter but their explanation is a little fast so I got doubts.

What are the differences between matter, dark matter and antimatter? Are they related? How they interact each other? Where can I see an example of this interaction? and also, in terms of percentage, how much of matter, dark matter and antimatter exist in the universe?

Thanks!


texts:
There surprisingly no proper definition of this term available on google. There are many research papers that are based on "finding the optical counterparts".

What exactly are optical counterparts and how to "find" them?


texts: What if Earth and Moon revolved around each other like Pluto and Charon?

texts: What would be different for us if Earth and Moon revolved around each other like Pluto and Charon do?


texts:
This answer to Compute Planet's Apparent Visual Magnitude reports the new work on planetary magnitudes, and contains an intriguing blurb about Neptune:

Neptune keeps getting brighter. No one knows why.

So I'd like to ask: How fast is Neptune getting brighter? When was was this first noticed and reported?


texts: How do one identify something they see in the sky?

texts:
I've always wondered how someone looking at the sky into the enormous amount of stars and other things out there figures out that whether it is an already identified object or is he looking at something new? Is there some sort of tools like latitude and longitude to pin point something in the sky or some catalog where you can see if this is an identified object and what it is called, how it looks like and where it is located and all?

As you might have guessed, I don't have any science background, just asking out of a common man's curiosity, so please be gentle with the jargon you use.


texts: How do I plot galactic coordinates using matplotlib and astropy in python?

texts:
I want to make a projection of millisecond pulsars on the galactic plane, much like this one from Sala et al. 2004:

I have tried several methods, and have not gotten anywhere. This is my current code, along with what it is producing:

import math as m
import numpy as np
import csv
import matplotlib.pyplot as plt
import pandas as pd
import astropy.coordinates as coord
import astropy.units as u
from astropy.io import ascii
from astropy.coordinates import SkyCoord

data = pd.read_csv('galacticwperiod.csv')

xarr = np.array(data.iloc[:,0])
yarr = np.array(data.iloc[:,1])
eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
gal = eq.galactic

#print(xarr)
#xarr = np.array(df.iloc[:,0])
#yarr = np.array(df.iloc[:,1])
#zarr = np.array(df.iloc[:,2])

#ra = coord.Angle(xarr[:], unit=u.hour)
#ra.degree
#ra = ra.wrap_at(180*u.degree)
#dec = coord.Angle(yarr[:], unit=u.deg)
#print(ra)

plt.figure(figsize=(6,5))
fig = plt.figure()
ax = fig.add_subplot(111, projection="aitoff")
plt.plot(gal.l.wrap_at(180*u.deg), gal.b.wrap_at(180*u.deg), linestyle='None')
ax.scatter(gal.l, gal.b, linestyle='None')

#ax.set_facecolor('xkcd:battleship grey')
#fig.patch.set_facecolor('white')
#ax.tick_params(axis='both', which='major', labelsize=10)
#ax.grid(color='b', linestyle='solid')
fig.show()
#plt.savefig('millisecondcoloraitoff.png', dpi=600)

Here are a few lines of the input file 'galacticwperiod.csv':

Gl,Gb
111.383,-54.849
305.898,-44.888
305.913,-44.877

This is the image it produces:

I am almost certain this is wrong because they are not distributed along the galactic plane, which they should be. The data I am using is from the ATNF Pulsar Catalog.

These are the sites I have already looked at, for reference:

https://stackoverflow.com/questions/33105898/astropy-matplotlib-and-plot-galactic-coordinates
https://astropy4scipy2014.readthedocs.io/_static/notebooks/06_Celestial_Coordinates_solutions.html

Any help with this would be greatly appreciated.


texts: Why is a giant planet around a tiny star unusual when binary stars are normal?

texts:
Inspired by this BBC article and the corresponding journal article about the M dwarf star GJ 3512.

  The Jupiter-like world is unusually large compared with its host star, contradicting a widely held idea about the way planets form.

On the other hand, binary stars are very common, comprising (according to Wikipedia) about one third of star systems.

Naively, the same processes that create binary star systems could be expected to also create star systems comprised of a single star and a Jupiter-like "failed star" - the only difference being that in the latter case the would-be second star wasn't large enough to ignite.

What is the difference between these scenarios?  What makes the small-star-and-large-planet, according to the standard models of planet formation, unlikely, when binary stars are common?

(In non-technical terms if possible, please.)


texts: Why did the distribution of asteroids discovered in 2010 have a radial modulation?

texts:
This answer links to one of Scott Manley's excellent asteroid videos Asteroid Discovery - 1970-2015 - 8K resolution. The animation highlights the positions of the meteors at the moment of their discovery, and by watching one can see the technology improve and notice patterns as instruments are pointed in different directions to avoid the light from the Sun and (at least sometimes) the Moon. (there is music, adjust volume accordingly)

There are often fan-shaped patterns showing the directions that more sensitive telescopes with modest fields of view are pointed.

However, I noticed that only during the year 2010, roughly between asteroid numbers 500,000 and 520,000 there are radial striations at certain distances from the Sun. I don't see this happen at any other time during the video.

Is this just a rendering artifact, or is it real? If real, what would cause the periodic radial modulation of sensitivity, and only in 2010?

note 1: YouTube allows for playback rates between 25% and 200%, and variable video resolutions. I found 25% and 1080p optimal for my current internet connection and screen.

note 2: For those with GIFs disabled, one image is a GIF.

note 3: 2nd image contains several cropped screenshots highlighting the "periodic radial modulation of sensitivity" in asteroid detection during 2010, for clarification purposes.


texts: How close would the sun have to get to a supermassive black hole to have Jupiter stripped and slingshot into interstellar space?

texts: Please make whatever assumptions are needed.  Assume sun approaches black hole on some sort of hyperbolic trajectory.  Assume size of supermassive black hole is same as the one at center of our galaxy.  How close would sun have to get in order for Jupiter to be “slingshot” into interstellar space.


texts: What is the difference between a depleted comet and an asteroid?

texts: When a periodic comet flies by the Sun enough times, it can become depleted of its volatile elements. Once it is depleted, what distinguishes it from an asteroid, other than records of past sightings showing that it used to have a tail when it got close to the Sun?


texts: Why can't supermassive black holes merge? (or can they?)

texts:
The CNet article Astronomers discover two supermassive black holes in a death spiral links to Discovery of a Close-separation Binary Quasar at the Heart of a z ~ 0.2 Merging Galaxy and Its Implications for Low-frequency Gravitational Waves (available in ArXiv) and says:

Supermassive black holes are usually found at the center of galaxies, including our own, and during a galaxy merger they end up beginning a dance of death, spinning around each other in a near-endless waltz, until finally merging. However, researchers are currently unclear as to the time it takes for black holes to merge -- or indeed, if they merge at all.
"It's a major embarrassment for astronomy that we don't know if supermassive black holes merge," said Jenny Greene, a professor of astrophysical sciences at Princeton and co-author of the study. "For everyone in black hole physics, observationally this is a long-standing puzzle that we need to solve."
This puzzle is dubbed the "final-parsec problem." Some astronomers believes that once two supermassive black holes get close enough together, reducing their distance to 1 parsec (3.2 light years), they may dance for an eternity.

Question: If it turns out that supermassive black holes can't merge, or have a tough time doing so, what might the reasons be?


texts: Recommendation for learning about stellar astrophysics

texts:
I would like to know which are the best books to learn about stellar astrophysics at (just) graduate level.

I have a basic formation in general astrophysics but I'm interested in learning about stars, specially their evolution and constitution. I also have a good mathematical formation so it should not be a problem (but I don't care if it focus on the ideas as long as it is a good book and make itself clear).

I found several books at the university library, but I don't know which of them are good (in my situation at least).


texts: Electron Degeneracy Pressure and the Pauli Exclusion Principle

texts: I have read that what keeps white dwarfs from gravitational collapse is electron degeneracy pressure. How does this pressure prevent further collapse, and how is it related to the Pauli Exclusion Principle?


texts:
I'm just curious if it is possible for a plant to survive on a large asteroid. Some say no, some say it depends.

With the scenario of an asteroid at about 1 AU from the Sun, receiving a similar amount of sunlight to Earth, and with wet soil or something similar.


texts: Would our galaxy appear as a spiral when seen from the Andromeda galaxy?

texts:
I'm wondering how our galaxy is tilted with respect to the Andromeda galaxy.

Seen from the Andromeda galaxy, what would our galaxy look like?

Would it appear as a spiral, or would it be seen edge-on, and appear long and thin?


texts:
I'm doing the following question:

  A star is observed with UBV magnitudes $m_u = 16.31$, $m_b = 14.52$,
  $m_v = 13.76$. Spectral analysis gives $M_{bol} = 7.31$, $BC = -1.02$, $(U − B)_0 = 1.222$. Determine the distance to the star.

As $m_u - m_b ≠ (U-B)_0$, I gathered that the distance modulus can't be used directly, since extinction is present. From the bolometric correction (BC), we can determine that the absolute visual magnitude $M_v = 8.33$.

After that, I'm unsure of what to do; normally I think in these questions you can obtain the visual extinction $A_v = 3.0E_{B-V}$, where $E_{B-V}$ is the color excess, calculated by $E_{B-V} = (m_b - m_v) - (B-V)_0$. Then, the visual extinction can be added to the distance modulus equation to account for extinction in the visual band, and then we can easily obtain distance.

But, while we know $m_b$, $m_v$, and $M_v$, we don't really have a way of obtaining $(B-V)_0$ to my knowledge, unless there's some way we can relate it to $(U-B)_0$. Any help would be much appreciated!


texts:
I have a spectral energy distribution (flux densities measured at several frequencies), and have fitted a 'greybody' model to these points.
I am confused about how I use this information to determine the luminosity of my source.
I think the integral of my fitted function gives me the total flux, but how is this related to the source's luminosity?


texts: From what distance would our sun have an angular diameter of 7 arc seconds?

texts: If an observer travels away from our sun, at what distance would our sun occupy 7 arc seconds of space?


texts: Stargazing (Tenerife) - different quality today and yesterday

texts:
I'm on the Tenerife island and (in the mountains above the clouds - 2300) the day before yesterday, I could clearly see the Milky Way and stars (1 am). So I went to the same place yesterday and the quality was much worse (at midnight).
I'm curious why there was such a difference and what parameters could cause this difference.

bigger moon?
different moon position?
sand from Sahara?

It's El Teide park cerca 2300 meters above the sea.
Is it worth to try again today? What parameters should I look up? It's like 2 hours of driving so I would like to be able to estimate the results in advance.


texts: Why does hydrogen ionization happen in HII regions? Why is the hydrogen there ionized?


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Articles summarizing a recent result about certain heavy element concentrations in stars near the galactic center all say that earlier reports about high levels in stellar atmospheres were the product of an "optical illusion". Examples include

UPI.com Study explains optical illusion at the center of the Milky Way
Science Daily Mystery at the center of the Milky Way solved
Phys.org High levels of scandium near the galaxy's giant black hole were illusory, astronomers find

At least Phys.org goes on to explain with real "science words":

  According to the new study, the lower temperatures of the giant stars helped to create the optical illusion that appeared in the measurements of spectral lines. Specifically, it means that the electrons in the elements behave differently at different temperatures, which in turn can be misleading when measuring the spectral lines of elements in different stars. The conclusion is the result of a close collaboration between astronomers and atomic physicists. (emphasis added)

Question: What was the "optical illusion" that led to erroneous metal concentrations in stellar atmospheres in the galactic center? Beyond just "electrons in the elements behave differently at different temperatures" (because how electrons behave in atoms at different temperatures should be at least fairly well understood by astronomers) what specifically is the source of the misunderstanding? Is it atomic physics, incorrect assumption just a mistake, something else?

The mention of "close collaboration between astronomers and atomic physicists" suggests that the answer will turn out to be fairly interesting!


texts: Is it possible that a planet originally full of water, and the solar winds decompose the water and hydrogen escapes out, then the oxygen combines with carbon at crust, just like Venus but the whole planets are mostly carbon dioxide and hence become a gas giant?


texts:
I am studying ways to make an anti-gravity machine (!!). I believe it can be done, or theoretically be done using gravity itself as a "force" that pushes outwards instead of inwards.
A similar question was asked here, which speaks about why objects in the universe are round, which as stated, is due to Gravity.
I understand Gravity is the strongest, most consistent "force" in the universe and to traverse it we have to push against it - jets, propulsion, etc.
Never the less, I am perplexed why gravity's forces make objects round - eventually or over time. Why can't it turn a planet into a cube for example? Why can't Gravity make objects flat, and perhaps round at the perimeter?
If I can answer this question, I am sure I can find a way to "bend" Gravity for my invention.


texts: Saint-Exupery describes visit to plateau where he easily finds several meteorites. Is this realistic?

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In "Wind, Sand and Stars" Antoine de Saint-Exupéry describes a visit to a plateau where it is extremely easy to discern meteorites from stones, since there are no stones.

I picked up one and then a second and then a third of these stones, finding them at about the rate of one stone to the acre. And here is where my adventure became magical, for in a striking foreshortening of time that embraced thousands of years, I had become the witness of this miserly rain from the stars. the marvel of marvels was that there on the rounded back of the planet, between this magnetic sheet and those stars, a human consciousness was present in which as in a mirror that rain could be reflected.

Is this realistic? Are there places on Earth where you can find meteorite on 5 minute walk?
edit: Judging by this NASA article looking for meteorites in Antarctica is relatively hard.


texts: How will planets behave in the night sky as seen by Mars colonists?

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Certainly among the first colonists on Mars there will be a few people interested in Astronomy enough to enjoy the night sky.

For them, familliar Mars will be missing and there will be a new, unfamiliar planet instead.

Planet by planet, how will their appearance and especially their behavior as seen from Mars differ from the way they behave in the night sky as seen from Earth?


texts: Does the Sun undergo any cycles or revolutions approximately every 24,000 years or 8.6 million years? In Hinduism it is believed that the Sun revolves in an elliptical orbit around a large mass at these rates.


texts: What if the black hole in the center of the galaxy grew faster?

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Imagine that hypothetically the black hole in the center of the milky way gradually increased in mass by for example 50% every year. That is exponential increase in mass.

Which visual effects would we see on for example Alpha Centauri, stars more distant and on M31? Red or blueshift? Lensing?

How about linear increase in mass?

Are there software I can use to visualize it as seen from earth?


texts: Starting a fire in a cold planet that was full of flammable gas

texts: What would happen to a cold planet that has a large amount of flammable gas, like Neptune, if we throw some fire into it? Will it burn, or would the flame be extinguished due to the cold?


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Why do proto stars on the Hayashi track get dimmer as they contract?

My expectation is that they would get hotter and brighter the more they contact. Why is this wrong?


texts: Spacing (2422 days) between solar eclipses - unrelated saros numbers

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I just made an observation that is very puzzling.  The observation concerns total solar eclipses and their spacing in time.

I know that the saros cycle dictates recurrence of same saros eclipses roughly every 18 yrs 11 days apart.  However, the two American eclipses (Aug 21, 2017 and Apr 8, 2024) are spaced by 2422 days.  These are saros 145 and 139 respectively - seemingly unrelated eclipses.

There is another pair of eclipses that are upcoming (Aug 2, 2027 and Mar 20, 2034), and they are also spaced exactly 2422 days apart.  These are saros 136 and 130 respectively.  Again, they are unrelated eclipses from each other - OK expected.  The only thing that these two pairs of eclipses have in common is that their paths CROSS each other 2422 days apart. So, here is my question:

Why are these two pairs of totally unrelated eclipses "dancing" with each other with exactly the same spacing of 2422 days?  What mathematical phenomena could account for this?

Thank you for your time.


texts: How much oxygen did the Warwick/Cambridge study find in the rocky debris around white dwarf GD 61?

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There are several articles out saying that a Warwick/Cambridge study of ultraviolet spectroscopy data from the Cosmic Origins Spectrograph on the Hubble Space Telescope found a large amount of oxygen in rocky debris around the white dwarf GD 61. They say that this amount of oxygen is indicative of a larger mass (asteroid), that was once composed of the rocky debris, which was 26% water.

(source: sciencedaily.com)
Image credit: copyright Mark A. Garlick, space-art.co.uk, University of Warwick and University of Cambridge
What I cannot seem to find is how much oxygen the spectroscopic examination found. Also, what methods did they use to determine how much water there would have been based on the level of oxygen?


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I was in Cuba from January 15th to the 22nd this year and there was a quarter moon. If I use a compass to explain the orientation of the quarter moon, it started in the north position around 4 pm. By 7 pm, it was in the east position, and by 11 pm, it was in the south position. Normally the quarter moon would start in the east and end in the east as it travels across the sky.

Could someone please explain this phenomenon?

Thanks!


texts: Why is the Sun so bright, but you can feel it far away?

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First of all, the Sun's surface temperature is only about 6000 K. Years ago I was working at at a melt work with furnace temperatures 0f about 2-3000 K; of course you could not be near a furnace without protection, when it was open, but you could still stay some 20-30 metres away from it. Likewise, a light bulb has no long reach, till it fades away. But how about the Sun? Why can experience it at 150 000 000 km away?

Besides, I wonder why space is not heated up, because the heat radiating from all the stars and galaxies is so enormous, and actually the matter density in space is so low. How, actually, can heat be absorbed in space? What temperature is measured in space - is it just the wavelengths of radiations, from which temperature is calculated?


texts: If an exoplanet transit we are seeing is 13000 light years away, are we seeing a 13000-year-old orbit?

texts: If a star is 13000 light years away, doesn't that mean we are seeing 13000-year-old light?  If it does, then does that mean when we discover a planet with dimming star light, we are seeing a planet that is 13000 years old?  Are we seeing a 13k-year-old orbit?


texts: My assignment is to identify how many planets are in this curve and what the orbital period is. I'm just confused on the way it looks, as most curves I've seen up to this date are more smooth and connected in appearance


texts: Has GAIA learned anything about General Relativity looking near Jupiter? (Gerry Gilmore: "oblate rotating mass moving in a deeper (Solar) potential")

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From Gerry Gilmore (2018) Gaia: 3-dimensional census of the Milky Way Galaxy

4.4 Fundamental physics
Relativistic effects are highly significant for Gaia measurement accuracy, with
tests of General Relativity being a significant driver from the very start of the project.
This established tight constraints on the mission. For example, sufficient modelling of
Newtonian aberration requires that the spacecraft orbit (Lissajous orbit around L2) is
quantified with a velocity accuracy of 1 mm/s. Finite light velocity effects lead to
position-dependent propagation delays in the field of view which must be accounted for.
Monopole light deflection (the famous 1.75arcsec solar limb effect first verified by
Eddington & Dyson in 1919) exceed the microarcsec level all-sky for the Sun, and up to
90 deg from Jupiter, significantly complicating the computational effort. Quadrupole
light bending is 240µas at the Jupiter limb, and is 1µas at 8 Jupiter radii. This allows a
special Gaia experiment – to quantify light bending by Jupiter, this test involving an
oblate rotating mass moving in a deeper (Solar) potential.

The Gaia mission
(also here):

Another possible experiment is to explore light bending of star images close to the limb of Jupiter to measure the quadrupole moment of the gravitational field of the giant planet.

Question(s):

Is looking so close to something as bright as Jupiter and still doing precision astrometry using GAIA possible? Has this been tried? Did it work?
Did they see "quadrupole light bending due to an "oblate rotating mass moving in a deeper (Solar) potential"?
GR tells us that the light will appear deflected near Jupiter and Jupiter's mass distribution is oblate because it rotates fast, so the deflection will be expected to have a quadrupole moment. But is any of that deflection due directly to the rotation, or would it be about the same for a static, non-rotating but oblate Jupiter? How far down the GR rabbit hole does this go?


texts: Could it be possible to detect exoplanets that have an abundance of iridescent minerals by analyzing their star's spectra over time as the angle in observation would lead to changes in absorbed wavelengths?


texts: Do black holes produce thermal radiation?

texts: Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by gauge–gravity duality, or not, as implied by Hawking's original calculation? If not, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure even exists?


texts: Does habitable zone take tidally locked bodies into account?

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Imagine a tidally locked planet orbiting a red dwarf, where habitability is not situated close to terminator zone, but on a "small" cap normal to incident starlight (zenith) with permanent average +15 Celsius over the year, variations through year depending on orbit's eccentricity.

If there was in our solar system one (maybe not spherical) tidally locked body orbiting some close to circular orbit around the sun, how far from the sun would it be to meet those +15C average zenith cap temperature? Does it make habitable zone larger than it is?


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I have an (observed) SED of a galaxy, and am attempting to match model SEDs to it. To do this I need to be able to see its shape better, which would involve some sort of smoothing of the spectrum. I would like to do this in Python. I've looked through scipy's various smoothing tools (like UnivariateSpline), but don't know if that's on the right track since that would only be some best fit to my spectral data.

I also used the Iris software to perform an interpolation of my spectrum with 1000 bins and the smoothing option turned on, but I do not understand the computational process behind this. How might I accomplish this in Python?


texts: Do asteroids have a gravitational field?

texts: I know that asteroids are huge chunks of rock, orbiting a solar system. Do asteroids have a gravitational field and do they gravitationally attract each other to form planets?


texts: Can you go blind if you look at the sunrise and sunset?

texts: Everyine knows its dangerous to look directly at the sun, but can you look safely at sunrises and sunsets without going blind? Because the uv radiaton and the light is much weaker. Or is it still very dangerous? How longcould you look at the sunrise or sunset without damaging your eyes? Will you be ok if you look at the sunrise, or sunset for a few seconds? Or is sungazing unsafe, no matter the time?


texts: Finding the precession period of folded light curves

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I was supposed to find the precession of a certain frequency in a time series. I tried breaking down the time series into smaller intervals and folded them over the average frequency I'm interested in. Then I plotted all of the folded light curves on top of each other.
I notice a slight oscillation in the shape of the light curve as I iterate through all of these light curves. I'm not sure how do I pin down the exact frequency of oscillation. The light curve doesn't resemble a pure sine wave so I don't think I can just fit a sinusoid and find the phase variation.
What all can I do to find the period of oscillation? Thanks a ton!


texts: How close to a host star can a tidally locked planet be and its dark side still maintain a moderate temperature?

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So, imagine an atmosphere-less planet, tidally locked to a sun-like star. How close to the star can the planet be before its dark side becomes too hot?
I imagine that at some point the rocks on its sunlit side will melt and evaporate so that the dark side would experience rocky precipitation. Would this be true?
Also, at some point the atmosphere of the star itself would engulf the planet.
But at which point would these effects make the conditions on the dark side unbearable?


texts: How do I see Deep Sky Objects in my telescope?

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I just bought a new telescope. I tried it out, and I even woke up at 3am when the moon set so there was light pollution. I have an 8 inch dobsonian telescope with a focal length of 1200 mm. For some reason, I only see points of light in the sky, except for the moon and Jupiter and the Orion Nebula.

My goal is to complete a messier marathon, but first I need to be able to see the Messier objects. So my question is, what is the best way to look for Deep Sky Objects with my telescope? And how much does light pollution get in the way of looking for them? For reference, I live in the northern United States.


texts: Does our current understanding of the mass of black hole only allow for it to be located at the singularity?

texts: Does the distribution of mass or density of mass inside the event horizon matter for a black hole to exist as we currently theorize? Could the mass inside the event horizon be distributed in any way at all, or have any density so long as it is dense enough to locate 100% of the mass inside the Schwarzschild radius?


texts: Aligning of 7 unidentified objects in the sky, what are they? What did I see?

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So this morning (4th December 2019, 0518hrs) I was out turning my horses out in the fields and noticed a strange arrangement of what I perceive to be stars, only I’ve never seen it before and wondered if someone might be able to explain or put forward some suggestions?

East North East in the sky there seemed to be 9 vertically aligned stars and 2 horizontally aligned stars with the lower second to last star - making an alternative shaped cross. On looking closer I realised they were moving (so definitely not stars then), altogether at first (downwards in the sky) but then as I carried on with what I was doing while watching as best I could, they’d skew a little in alignment then move back - within 10 mins I’d lost nearly all of them and could only see 2.

Anybody have any idea what these may be? They were white in colour to the naked eye, as bright as the surrounding stars except for one just to the right hand corner of the top ‘object’ (which was slightly brighter) and we’re moving quickly enough for me to notice but not enough to stop me immediately questioning my eyes.

Photo taken with an IPhone XS Max, no filter and extremely limited light pollution in the sky.


texts: What is the apparent magnitude limit for the naked eye?

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If you only read the question, you will answer: ''That depends on the light pollution''. Yes, it does, but if you are in the darkest night sky (Bortle class 1), what magnitude would have the faintest star? In the Wikipedia article about the magnitude, it's written that +6.0 would be the typical limit, but in the article about the Bortle scale, I saw the fainter magnitude +8.0. So, what is the real limit?

If the question has been already asked, please give me the link.


texts: Is the moon rotation at complete standstill?

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Moon always faces Earth with one side, its spin period equal its orbital period - this is what we learn at school.

But I know Nature rarely operates in absolutes. I'd find it hard to believe Moon wouldn't spin relative to Earth at least very slowly. I searched but found no data other than "doesn't", so I'd like to ask - does the Moon stay turned the same side precisely - or do, over many years, new features show at one edge? If it does, how long would it take to make a full turn?


texts: Is there or will there be any search after a primordial black hole 500 au from the Sun?

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Since 2019 it is proposed that the hypothetical Planet IX may in fact be a primordial black hole. Black holes are being looked after differently than planets. You can look for a gravitational lens or for an accretion disc. However, the black hole theory doesn't seem to be taken serious by many.
Is there any ongoing search for such black hole rather than a planet, or are there any plans to look after a primordial black hole if the planet theory would get disproved?


texts: Would an extremely bright star produce same kind of shadow band effect as seen in solar eclipses just before totality?

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This answer explains that the shimmering patterns seen on surfaces at the moment before totality of a solar eclipse are called shadow bands, and that the phenomenon is closely related to the twinkling appearance of stars (but not angularly larger planets) through the Earth's atmosphere.

If for some reason there happened to be a bright star close enough to Earth that its nighttime illumination visible on surfaces, but small enough in angular width that it twinkled, would we see shadow bands at night, every night when the star was out?

As there's a good chance the answer is "yes" to the hypothetical, I'd like to ask if this situation is in fact possible, or if a star were close enough to be bright enough to visibly illuminate a white surface at night, would it then be large enough in angular extent that it wouldn't twinkle any more?

The pattern from the Eclipse can be seen in the Smarter Every Day video Space Station Transiting 2017 ECLIPSE, My Brain Stopped Working - Smarter Every Day 175 at about 04:30.


texts: Characteristics of the first planets in the Universe?

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What would have the very first planets looked like, based on their most likely chemical compositions?

For example:

Were they mostly grey gas giants with atmospheres of hydrogen and helium, coalescing around the first Population II stars?
Were they blue gas giants, enriched with water vapour?
Were they brownish-red and grey, like our own Jupiter and Saturn?
Were they a heterogeneous mix of all sizes, colours, and chemical compositions that included rocky and gas giants?

What papers support research into this area?

https://iopscience.iop.org/article/10.1088/0004-637X/751/2/81/meta
https://www.mdpi.com/2076-3263/9/3/105
https://arxiv.org/pdf/1805.00023.pdf

Note: By first planets I mean the first planets that most likely existed in the Universe. In The habitable epoch of the early Universe, for example, Loeb suggests that a mere 13.5 ± 3.5 Myr after the Hot Big Bang could have witnessed the first viable supernova followed by planet formation. Another paper (can't find it) suggested that gas giants could have formed from the primordial hydrogen and helium soup before reionization, without needing any stars.

Related:

http://www.sci-news.com/astronomy/science-helium-rich-gas-giants-02905.html
https://en.wikipedia.org/wiki/Sudarsky%27s_gas_giant_classification
https://www.nasa.gov/centers/goddard/news/topstory/2003/0709hstssu.html
https://www.space.com/17441-universe-heavy-metals-planet-formation.html
https://earthsky.org/space/how-and-when-did-the-first-planets-form-in-our-universe
https://medium.com/starts-with-a-bang/ask-ethan-how-fast-could-life-have-arisen-in-the-universe-863bd17b92ca
https://www.nationalgeographic.com/news/2012/3/120326-oldest-planets-found-jupiter-big-bang-space-science/


texts: Is the data gathered from telescopes "open source"? If so, is there any website that group it all?


texts: Black Hole Collision & Gravitational Waves

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The Gravitational waves detected by LIGO on 14th September 2015 are attributed to a collision of two black holes, which had been rotating near the speed of light around each other just before the actual collision. The collision happened 1.3 billion years ago, approximately 1.3 billion light-years away and apparently, the energy of the collision exceeded the combined energy of all stars in the observable universe.
LIGO was able to detect the deformation in the fabric of space induced by the gravitational wave caused by the collision, even though the deformation was the magnitude of a fraction of an atomic radius. The reason why the deformation was so "small" by the time the wave had reached planet Earth was due to the relatively large distance of the collision from Earth.
Question: could someone qualified give an estimate of the magnitude of the space-time deformation we would experience on planet Earth, had the collision happened closer to us? I.e. how close would the collision have to happen for the space-time deformation to be millimeters? What about meters? At what magnitude of the deformation would the gravitational wave be dangerous or lethal for humans on planet Earth?
Bryan Greene described the gravitational wave deformation of space-time as a temporary "shrinkage" or "compression" of Earth (and everything on it). Am I right to assume that being compressed by even 1 centimeter could potentially be lethal for all live on Earth?


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A few weeks ago my daughter phoned me to say she was looking at a very large bright object in the sky in the direction of the Preseli hills. She wondered if it was a planet. I went out to have a look and sure enough this was much brighter and much larger than a star or planet. I looked at it for several minutes and quite suddenly it just, for want of a better phrase, went out. Just completely gone.

Last night I saw something very similar. Again it was much larger than a star but this time appeared yellowish in colour. I kept looking as I was doing a few small outdoor jobs and then, although I didn't see it happen this time, it suddenly disappeared. I am mystified as to what these lights could have been. Looked on internet and hoped someone else may have seen them but no luck.


texts:
I always thought the only fuel for a star was hydrogen, which is fused into helium.

But while reading some questions and answers here in ASO, I saw phrases like "This balance stays relatively stable until the star runs out of whatever its current fuel is".

Besides hydrogen, what are the possible fuels in a star’s core?


texts: How long would it take to collect a certain amount of photons?

texts:
So, I've received this question to solve, it's not a very advanced one, though advanced enough to cause headaches.

The situation is as follows:

  Say, you are observing with a telescope with an effective area of 44000 $cm^2$. Assume that the total efficiency through the atmosphere, the telescope, optics and the filter is 50%. How long would it take to collect 80 photons from a source with V=20?

I have little to no idea how to tackle this problem. Can someone help? I've had lectures about spectral flux density, and I'd assume you'll have to use it here.

Thanks in advance!

Adam.


texts: What type of energy is escaping from black-hole's poles?

texts:
Following this article, it is stated that:

  As a star drifts too close to a supermassive black hole, intense tidal
  stresses rip the star to shreds. As this happens, the shredded
  material will be dragged into the black hole’s accretion disk — a hot
  disk of gas that is gradually pulled into the black hole’s event
  horizon, bulking up the black hole’s mass or blasted as energetic
  jets from its poles.

Questions are,

What is the energy type that escapes from the poles?
What is the magnitude of the power?
Can this energy be potentially harnessed?
How far into space do these energy jets blow?

Also, if you feel like discussing this: if mass m goes into the black hole and comes out as energy E from the poles, assuming the speed of light c has no play in this transaction of going in or out, what happened to $c^2$ from ? Could a black hole be hypothetically faster than light? Or maybe the reverse direction of light? (I even got confused from this thought...)

So, if those jets are not pure energy, but are energized matter,

What type/kind of matter it would be? (gas? solid? liquid?)
Is all the matter that comes in, also goes out? I am thinking if the black hole works like a car's engine - does it have conversion efficiency?
Can this matter be cultivated, harnessed, or converted back to what it was or something new all together?

.

This question is presented here after it was asked on the space stackexchange. I want to leave it there because it has some relevant discussion to this question overall.


texts:
How to define moonless night? There is no moon at all during some night?
How to calculate and know whether the moon would appear during one night?

If the lunar phase is full moon, is it possible that the moon will not appear ?


texts:
Last night (January 21, 2015, possibly 5:30 UTC), from my house in northern California, I saw a moving point of light in the sky, going west to east, crossing about 120 degrees of sky in 5 seconds or so.
It didn't look like my experience of meteors - thicker and slower, and quite bright. I think it survived until it was out of sight entirely, rather than burning out.
Aerospace.org's reentry predictions doesn't show anything for that time.
I've seen ISS and other satellites; this was a lot larger and brighter, and my favorite satellite pass schedule also shows nothing.
Was this more likely a rocket body re-entry, or a large meteor?


texts: Does a photon honor the causality principle?

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We know that the faster you move, the slower your time is perceived from an external observer and the space along the direction of the movement shrinks.

Photons, having no mass, move at the speed of light c (in the vacuum) and for them the space has no meaning because it's collapsed into one single point (edit: better say into a single plane, because only the direction along the path collapses). Actually, they don't "move", just "are" at the same time in all places along their path.

In the same way, for them time has no sense. Because they don't "move" they take 0 time to reach their destination.

This seems to be odds we the causality principle, but of course I'm wrong.
I try to explain it with a simple example. Let say a photon is emitted from a very far star millions of years ago and I receive it in my eye now. It means its path was started on that star and ended in my retina.

It took millions of years, for us, and 0 seconds from its point of view.
So, I apologize for the stupid question, but how it was able to "know" that my retina will be there? If I was few cm apart, perhaps it would have continued for other million of years until it will eventually reach something other. But for it, the time is still 0.

How this could be explained?

And, we know the speed of light is less than c in other media. But it's not clear to me if from the point of view of a photon changes something. For it, the space and time are still 0? As an external observer I can say it still moves as fast as it can. From its point of view it's always in all points of its path.

Is it correct?


texts: how to find the approximate [stellar radius]? (distance from center to most distant star) of a circular flat disk galaxy based on its stellar mass?

texts:
I'm doing a high school programming project on drake's equation (simple galactic model) and wondered if there was a (simplified) way to find the radius (measured by the distance between the center and furthest star) of a galaxy based on its stellar mass. I found a few websites but I don't understand many of the terms and symbols in the equations shown. would love a simplified equation if possible.
In the project, I am modeling a 2d circular disk galaxy with no rotation.
I have a Y10 level understanding of maths.


texts: Is it possible to see stars that are gravitationally lensed by the sun from the ground?

texts:
Roughly 100 years ago, Eddington confirmed Einstein’s general relativity theory by photographing stars behind the obscured Sun during a total eclipse.

A coronograph is a device that mimics what happens during an eclipse and blocks the light that comes from a star so that nearby objects (which would otherwise be hidden by the star's glare) can be seen. This device was invented in 1931 by Lyot, 12 years after the aforementioned eclipse.

Would it be possible to use a coronograph with a ground-based telescope to reproduce the observation done by Eddington in 1919? Or would the light from the Sun scattered in the atmosphere be enough to hide the stars for which the gravitational lensing is visible?


texts: How do you calculate the lookback time distance to a given galaxy?

texts:
By lookback time distance I mean the same as Light travel distance.
Can you calculate it by just knowing its redshift?